ENH: add kwarg to generate_locations to filter for activities

[bifbof]

closes #427

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.05% 🎉

Comparison is base (eb7d6ce) 92.31% compared to head (8623598) 92.37%.
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #493      +/-   ##
==========================================
+ Coverage   92.31%   92.37%   +0.05%     
==========================================
  Files          39       39              
  Lines        2004     2019      +15     
  Branches      340      344       +4     
==========================================
+ Hits         1850     1865      +15     
  Misses        133      133              
  Partials       21       21              

Files Changed	Coverage Δ
trackintel/preprocessing/staypoints.py	99.08% <100.00%> (+0.06%)	⬆️

... and 3 files with indirect coverage changes

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